Recently, as optical materials, plastics have been predominantly used because they are light-weight and resistant to fracture in comparison with glasses and readily stainable. Commonly used plastic materials for optical applications include polyethyleneglycol bisallyl carbonates, polymethyl methacrylates, alicyclic polyolefins and polycarbonate resins. However, these materials have a refractive index of 1.6 or less, so that when they are used for coating or bonding of a material having a high refractive index, the properties of the object adhered cannot be fully utilized and furthermore, when they are used for an optical lens, a lens thickness becomes larger. Since a plastic material having a higher refractive index generally tends to have a lower Abbe's number and a larger chromatic aberration, an observed object looks tinted and blurred. Thus, a material having a high refractive index and a high Abbe's number has been needed as an optical resin.
As such an optical resin, Patent document 1 has proposed a polythiourethane obtained by reacting a polythiol and a polyisocyanate and Patent document 2 has further proposed such a polythiourethane prepared from a polythiol with a higher sulfur content. Since these polythiourethanes generally have a higher refractive index of 1.6 or more and a higher Abbe's number of 30 or more, many of these have been used as a recent thin and light optical lens.
In seeking a material having a further higher refractive index, there has been proposed a polythioether as a ring-opening polymer of a polyepisulfide described in, for example, Patent document 3 or Patent document 4. Since a polythioether has a higher refractive index of 1.65 or higher to 1.7 or higher and a higher Abbe's number of 30 or more, it has contributed to thinning of an optical lens or a prism.
However, a polythioether tends to have a smaller bending distortion, leading to its fragility. There has been, therefore, needed such a material having a higher refractive index and a higher Abbe's number, which is endowed with flexibility to a bending stress. For giving flexibility to a bending stress, polythioethers have been prepared using a polythiol or polyamine as a resin modifier (for example, Patent documents 5, 6 and 7, etc. (Japanese Laid-open Patent Publication No. 1997-255781, 1999-322930 and 1999-258402), but these have lower heat resistance (glass transition temperature).
Patent document 1: Japanese Examined Patent Publication No. 1992-58489;
Patent document 2: Japanese Laid-open Patent Publication No. 1993-148340;
Patent document 3: Japanese Laid-open Patent Publication No. 1997-71580;
Patent document 4: Japanese Laid-open Patent Publication No. 1997-110979;
Patent document 5: Japanese Laid-open Patent Publication No. 1997-255781;
Patent document 6: Japanese Laid-open Patent Publication No. 1999-322930;
Patent document 7: Japanese Laid-open Patent Publication No. 1999-258402.